Fabrication of the First US ITER TF Conductor Sample for
Qualification in SULTAN Facility
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Gung, C.-y. et al. “Fabrication of the First US ITER TF Conductor
Sample for Qualification in SULTAN Facility.” Applied
Superconductivity, IEEE Transactions on 19.3 (2009): 14741477. © 2009 IEEE
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Fabrication of the First US ITER TF Conductor
Sample for Qualification in SULTAN Facility
Chen-yu Gung, Nicolai N. Martovetsky, Daniel R. Hatfield, John R. Miller, Ji-Hyun Kim, and Joel H. Schultz
Abstract—A pair of 3.5 m long ITER TF size straight conductors
has been fabricated into a conductor short sample and submitted
to the SULTAN facility at CRPP for cold test. The sample used
a triplet-based cabling pattern in one leg and a septuplet-based
in the other. The legs had different cabling pattern and strand
diameters, but the same void fraction. To assure the accuracy in
measurement of the conductor current sharing temperature, it is
important to have uniform current distribution in the cable, which
requires uniformly low interstrand resistivity in the joint. In the
present sample, the cable/subcable wraps and the chrome plating
on all strands were removed from the cable in the termination,
followed by compacting and heat treating the termination in a
Glidcop sleeve. To improve current transfer, the sintered termination was further filled with soft solder before it was soldered
to the copper profile. To clarify the effectiveness of short sample
instrumentation, the sample was equipped with enhanced number
of sensors and with sensor mounts penetrating the conductor
jacket for the thermometers and voltage taps positioned in the
high field zone. This paper presents the experiences in sample
fabrication and instrumentation, and outlines the parameters
used in the key processes.
Index Terms—Superconducting cables, superconducting magnets, thermonuclear fusion.
NUMBER of 3.5 m long straight ITER TF conductor
samples, some with different fabrication processes of the
Nb3Sn strands, have been recently fabricated by different ITER
Domestic Agents (DAs) and tested in SULTAN to evaluate
the effects of cabling pitch and conductor void fraction on the
Tcs, Ic, and degradation by cyclic load [1]–[3]. The test results
showed that the differentiation in the Tcs was not consistently
correlated with the design of the strand and the featured modification in the cable construction [2]. In a few conductors, the
measured results were significantly affected by the transverse
voltage component.
The first US TF conductor sample TFUS1 fabricated for test
in SULTAN was designed in late 2006, which consisted of a
baseline and an alternate conductor legs. The sample design
window as well as the baseline conductor leg using a tripletbased cabling pattern followed the ITER specification for the TF
conductor sample [4]. The alternate leg designed by the USDA
Manuscript received August 26, 2008. First published June 23, 2009; current
version published July 15, 2009. This work was supported by the US ITER
C. Gung, J.-H. Kim and J. H. Schultz are with the MIT Plasma Science and
Fusion Center, Cambridge, MA 02139, USA (e-mail: [email protected]).
N. N. Martovetsky, D. R. Hatfield, and J. R. Miller are with the US ITER
Office, Oak Ridge, TN 37380, USA (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2009.2018203
had a septuplet-based cabling pattern [5]. Similar cabling pattern with fewer strands and cabling stages was used to construct
NHMFL 45 T hybrid magnet [6], [7].
The purposes of fabrication and test of the present sample
are to compare the performances of conductors constructed
with different cabling patterns, and to investigate the uniformity of current distribution in the present sample fabricated
with sinter-bonded and solder-filled cable in the conductor
termination. This paper focuses on the fabrication of the conductor sample [8]. The test results of TFUS1 are presented in a
companion paper in these proceedings [9].
Table I summarizes the construction of the as-built conductor
sample legs [5]. Both baseline and alternate cables were made of
internal-tin Nb3Sn strands with the same strand cross sectional
construction. A smaller strand diameter was used in the alternate
conductor in order to achieve the same target void fraction of
32.6% in the final cable.
Both conductors used a perforated stainless steel (SS) seam
tube with a wall thickness of 1 mm and 30% opening as the central coolant channel. The conductor jacket was made of modi-
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fied 316LN seam tube with a wall thickness of 1.6 mm. The
same jacket material had been used to fabricate the CICCs in the
NHMFL hybrid magnet [6]. Mechanical properties of the coldworked and aged base-metal and weld of the modified 316LN
have been tested at 300 K and 4 K [10], which meet the ITER
TF jacket requirements.
A. Sample Termination
The TFUS1 adopts termination used in the CS model coil
inner module (CSMC-IM) [11], [12] modified to achieve uniformly low inter-strand resistivity. After removing the jacket,
all cable wraps and Cr plating on all strands are removed before compaction. The cable in the termination is compacted inside a leak-tight braze assembled Glidcop/Monel tubular sleeve.
Cu-Cu sinter-bonds among neighboring strands and at the interface between the cable and the Glidcop sleeve are formed during
reaction heat treatment. Furthermore, the sintered cable in the
termination is solder-filled to increase interstrand contact area
that benefits the uniformity of current distribution.
B. Restrain of Cable Slippage
The sample is designed to utilize the friction between the
cable and the Glidcop sleeve in the compacted termination to
restrain differential thermal contraction of the cable against the
jacket in the straight conductor length during the cool-down
phase in the reaction heat treatment. The terminations are compacted before heat treatment to a target void fraction of 20% and
with a compacted length 450 mm. The compacted terminal
sleeve is permanently welded to the conductor jacket before reaction. The friction between the cable and the sleeve is preserved
in the post-reaction sample handling and cold test. Differential
thermal strain should be constrained in the reacted sample leg.
Crimping short section of CICC to a void fraction of 20% near
the conductor-termination transition [4] is not used.
C. Instrumentation
The sample is instrumented with enhanced numbers of
voltage taps and Cernox thermometers. Sensor positioned in
the SULTAN high field zone is installed on a penetrating sensor
mount, which will be described in a later section. The layout of
the sensors along the conductor leg is detailed in [9].
A. Sample Leg
1) Conductor Length: Each of the conductor leg was cut to
a final length of 3460 mm. The jacket and wrap were removed
from each termination with a length of 520 mm. The cable twist
pitch was preserved with a number of SS band clamps.
2) Chrome Plating Removal From Termination: Removing
chrome plating from all strands in the termination was achieved
by dipping the cable in an HCl acid bath with a concentration of
18–20 w% [2]. All SS band clamps on the cable, except the one
at free end, were removed. A 40 kHz, 500 W ultrasonic transducer was used to agitate the bath for 30 minutes to promote
chrome removal from strands buried deep inside the cable. The
cable region was purged with nitrogen gas throughout the dipping process. After chrome removal, the cable was rinsed with
distilled water and neutralized with saturated NaHCO3 solution.
The copper surface was brightened with 50% diluted Kester
645 flux. This was followed by dipping the cable in water and
methanol baths, and purging it with nitrogen gas till it dried. The
SS band clamps were reinstalled on the dry cable after slightly
tightening up the cable to recover the original pitch length. The
end face of the chrome removed cable was TIG welded to prevent tin leakage during heat treatment.
3) Glidcop-Monel Sleeve: The terminal sleeve consisted of
a Monel 400 cap, a Glidcop Al-15 tube with a wall thickness of
3 mm, and a short length Monel 400 transition piece. The components were assembled and furnace brazed with 35Au65Cu
alloy in a reduction atmosphere. The brazed assembly was
leak checked and inspected with radiography to assure that
unbounded area does not cross the full width of the shear joint,
and the overall unbounded area is less than 10% of the joint area.
4) Compaction of Conductor Termination: The accepted
Glidcop-Monel sleeve was mounted over the chrome-removed
cable in the termination. A hydraulic pointing machine with
a compaction die of 42.8 mm dia. was used to form the termination. The void fraction in a compacted termination was
estimated as 19%. Table II shows the dimensions of the sample
legs with compacted terminations. Less than 0.5% inconsistency in the length of the CICC was due to the discrepancy in
the initial engagement of the cable end in the Glidcop tube.
The bend of 0.33 mm in the 470 mm long compacted
termination was caused by the axial misalignment between the
Glidcop tube and the Monel transition in the brazed assembly.
Intermittent welding process was used to join the Monel to the
conductor jacket, which was pre-qualified to maintain the temperature of the cable under the weld joint below 200 . Alignment between the conductor and the termination was constantly
measured and corrected during the welding process.
B. Pointing Sensor Mount
Figs. 1(a), 1(b) show the design and fabrication of the penetrating sensor mount. A SS tube (6.4 mm 9.5 mm) was fusion
welded to a 16 mm OD 1.4 mm thick SS washer that was prebent to a radius conforming to the jacket outer surface. The OF
Cu core was torched brazed in the SS tube using 72Ag28Cu alloy
. The upper cylinder was machined to a plateau
2.6 mm above the washer and with a 1.6 mm deep flat bottom
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Fig. 1. (a) Design of the penetrating sensor mount with a Cu tip pointing 2
mm into the cable; (b) top and bottom views of the penetrating sensor mount;
(c) removing jacket and cable wraps at the sensor location; (d) welded sensor
mount after heat treatment trial; (e) Cernox installed in the Cu groove with the
lead wires thermally anchored around the mount.
Fig. 3. Solder filling system and connections to sample termination.
Fig. 2. Reaction heat treatment schedule for TFUS1 conductors.
groove across the diameter to house the Cernox-SD thermometer.
The Cu core on the opposite side was machined into a pointing
tip with a length penetrating into the cable region by 2 mm.
At sensor location (Fig. 1(c)), the jacket was machined with
a 6.4 mm dia. hole and the outer and sub-cable wraps were removed. No strand was broken during this process. Each sensor
mount was installed with the Cu tip in contact with the strands
in the cable. The washer flange was then fusion welded to the
jacket before heat treatment (Fig. 1(d)). Again, intermittent weld
was used to prevent overheating the cable.
C. Reaction Heat Treatment
The conductor legs were heat treated in a leak-tight retort
purged with purified Ar gas. Each cable region was purged with
an independent Ar circuit. The oxygen and water contents at the
inlet of the purge gas were both below 0.05 PPM throughout
the heat treatment. Four Jc barrels and one RRR samples were
co-heat-treated with the conductors.
Fig. 2 shows the reaction heat treatment schedule defined by
the strand manufacturer Luvata. The temperature distribution
along the conductor length was within
at any stage. The
lag time of the load temperature during temperature ramping
was less than 30 minutes at
and less than 15 minutes
D. Solder Filling Sinter-Bonded Cable in the Termination
All four terminations in the reacted TFUS1 sample legs were
solder-filled. To allow coolant flowing through the termination,
a 5.6 mm OD 0.4 mm wall Cu tube was inserted inside the
coolant channel before the process. In the terminations for
making the joint, the upper end of the Cu tube was pre-brazed
with a 768 mm long SS rod that prevents the coolant from
streaming through the central coolant channel in the high field
zone [4]. Four 25 mm L 3.2 mm W openings were machined
on the Cu tube near the braze joint, 13 mm above the neck
of the compacted termination, as the coolant outlets to the
conductor region. The Cu reentry tube was left in the central
coolant channel after solder filling.
Both flux (Kester 817) and solder (63Sn37Pb) were fed into
the cable region using gravity siphon [2], [7]. In order to remove gas trapped in the cavities in the sinter-bonded cable, the
conductor and the system were evacuated during fluxing and
solder-filling processes. Flux was introduced into the termination with the cable warmed up to
. The conductor was
back-filled with N2 gas before rinsing out the flux with 1% HCl
solution and raising the termination temperature to
The conductor was re-evacuated at a higher temperature.
As shown in Fig. 3, the bottom of a 22 mm dia. SS solder pot
was welded with a 13 mm dia. SS feed tube that tapped into a
coax tubing. The inner bottom of the solder pot was positioned
at 12 mm below the neck of the compacted termination to prevent the molten solder from blocking the Cu reentry tube. The
solder pot was sealed after loaded with solder alloy, evacuated
and then heated with a propane torch when the temperatures in
the termination and feed tube reached 250 . The discrete level
sensor in the solder pot, made of isolated stainless steel rods of
different lengths, was used to detect the melting of the solder
alloy and measure the level of the molten solder.
The amount of solder filling in the termination was estimated
from the difference in weights between the initial quantity and
that left outside the termination after solder filling. As seen in
Table III, 90%–95% of cable void space had been solder-filled.
At completion of the solder filling process, the coax feed tube
was cut off from the termination. A final cap was welded to the termination after machining out the solder inside the brazed Monel
cap between the open end and 6 mm above the weld line.
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E. Sample Terminations and Joint
A round bottom channel was machined to one side of a flat
OF copper plate, which was soldered to the compacted Glidcop
length to form the sample termination. The sample lower joint
was assembled by soldering the conductor terminations to an
OF copper bar pre-machined with mating grooves.
The bend in the compacted termination was sanded to below
0.13 mm in the soldering direction. The terminations and the Cu
profiles were pre-tinned with 97In3Ag alloy
Zinc chloride based soldering paste and solder foil (0.25 mm
thick) were used to solder the terminations and the joint. The
and 165 .
soldering temperatures were both between 150
The averaged solder layer in the joint was estimated as 0.1 mm
based on the weights of the solder alloy.
Fig. 4. (a) Conductor surfaces between adjacent sensors were electrical insulated from the conductor clamps with multilayer of G-10 sheaths and SS foils;
(b) plastic angles were installed on the sample corners to insulate the clamps
from the test well.
The first US TF conductor sample constructed with two
conductors using distinct cabling patterns, sinter-bonded then
solder-filled terminations, and enhanced instrumentation has
been successfully fabricated. The test results presented in a
companion paper [9] indicate that the featured termination
and joint built in the TFUS1 conductor sample could improve
the quality of the short sample test results and benefit the
qualification of the ITER conductors.
F. Instrumentation
All sensors positioned in the high field zone were installed
after the lower joint was soldered. To accommodate 150 diagnostic wires [9] in a design radial clearance of 0.85 mm between the conductor jacket and the clamp, Formvar coated 32
AWG lead wires were soldered directly to the jacket, the sensor
mounts, and the Cernox thermometers.
As shown in Fig. 1(a), the voltage tap at the penetrating sensor
mount was installed on the outer surface of the SS tube to avoid
interference with mounting of the Cernox. The Cernox-SD was
bonded to the bottom of the Cu groove with VGE-7031 varnish. Fig. 1(e) shows that the Cernox lead wires were coiled
and expoxied around the penetrating sensor mount for thermal
anchoring. Apiezon-N was used to improve thermal contact between the Cernox and the Cu surface. Kapton wedge and tape
were then used to secure and insulate the sensors.
G. Insulation and Clamping
The conductor was insulated from the clamp with 0.25 mm
thick G-10 sheath and 0.02 mm thick SS foil wrapping around
the conductor to fill in the radial clearance to the clamp. Fig. 4(a)
shows that the multilayer insulation was tailored into different
lengths to cover the conductor surfaces between the adjacent
sensors. Half of the lead wires were routed from the inboard
to the outboard of the sample for connection to the diagnostic
cable. Care was taken to assure that the radial build of the lead
wire was less than the thickness of the insulation. After the conductor legs were secured with heavy SS clamps, twisted Teflon
insulated wires were used to connect the lead wires to the electrical connectors [4]. The coolant tubing were welded to the top
and bottom ends of each sample leg, and insulated with 0.1 mm
thick Kapton tape. As seen in Fig. 4(b), plastic angles were installed along the corners of the clamps to insulate the sample
from the facility test well. The finished sample was pressurized
then leak checked to meet the requirements in [4].
The authors thank Drs. P. Bruzzone and B. Stepanov at CRPP
for their valuable advice and assistance on sample design and
preparation, and SUPERCON Inc, MA for their kindly corporation in chrome removal process. C.Y. Gung thanks D. Tracey
and MIT Alcator personnel for their excellent technical support
during this sample fabrication.
[1] N. Mitchell et al., “The ITER magnet system,” IEEE Trans. Appl. Superconductivity, vol. 18, pp. 435–440, 2008.
[2] P. Bruzzone et al., “Results of a new generation of ITER TF conductor
samples in SULTAN,” IEEE Trans. Appl. Superconductivity, vol. 18,
pp. 459–462, 2008.
[3] D. Bessette and N. Mitchell, “Review of the results of the ITER toroidal
field conductor R&D and qualification,” IEEE Trans. Appl. Superconductivity, vol. 18, pp. 1109–1113, 2008.
[4] Specification of the Sultan Short Sample for the Qualification of the
ITER Conductors revision V-03, Nov. 2006, unpublished.
[5] N. N. Martovetsky, TFUS1 TableR1.doc Jan. 12, 2008, unpublished.
[6] J. R. Miller, “The NHMFL 45-T hybrid magnet system: Past, present,
and future,” IEEE Trans. Appl. Superconductivity, vol. 13, pp.
1385–1390, 2003.
[7] T. A. Painter et al., “Progress in the manufacture of the cable-in-conduit Nb3Sn outsert coils for the 45 Tesla hybrid magnet,” IEEE Trans.
Magnetics, vol. 30, pp. 2204–2207, 1994.
[8] C. Y. Gung and J. H. Kim, TFUS1 Short Sample Fabrication Report
MIT internal report, ITER-USMIT-GUNG-20080420-01, April 2008,
[9] N. N. Martovetsky et al., “Test results of the first US ITER TF conductors in SULTAN,” in , in these proceedings.
[10] J. H. Kim and J. Feng, “The 4 K mechanical properties of modified
SS316LN for jacket materials in superconducting fusion magnets,”
Adv. Cryo. Eng. (Material), vol. 54, pp. 92–99, 2008.
[11] C. Y. Gung et al., “Design and manufacture of the US-ITER prepototype joint sample,” IEEE Trans. Appl. Superconductivity, vol. 7, pp.
469–472, 1997.
[12] R. Jayakumar et al., “Fabrication of ITER central solenoid model
coil inner module,” IEEE Trans. Appl. Superconductivity, vol. 7, pp.
981–984, 1997.
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Fabrication of the First US ITER TF Conductor Sample for